This invention relates to measurement of electrostatic fields associated with static charge on semiconductor wafers, and more particularly to apparatus for sensing such electrostatic fields on semiconductor wafers during transitions into and out of semiconductor processing stations.
Contemporary semiconductor facilities are designed to process wafers of about 300 mm diameter. Static charge on wafers creates problems by attracting contamination particles, and can also cause electrostatic discharge (ESD) which damages the sub-micron size features of the semiconductor devices fabricated on the wafers. The wafer accumulates static charge from contact and separation (triboelectric charging) that occurs during normal process-chamber activity and from contact with moving equipment such as robotic loaders. Due to the material of the wafer (oxide-coated silicon), the static charges remain isolated and the charge distribution on both sides of the wafer is often not uniform, thus requiring several measurement points to characterize charge on the wafer. There is difficulty in measuring electrostatic fields associated with static charge on a wafer without interfering with the wafer processing.
At the beginning and end of each processing step in the semiconductor manufacturing process, the wafer is placed in a specialized carrier called a Front Opening Universal Pod (FOUP). Some possible methods for taking charge measurements on semiconductor wafers in this environment include:
1. Stopping the robot to measure the field at several points along the surface of the wafer with a fieldmeter or charge sensors while the wafer is being transported to or from a process chamber; or
2. Using charge sensors to measure several points on the surface of the wafer while at rest in the FOUP.
The first method of stopping the robot with the wafer in mid-transport to measure with a fieldmeter the electric field caused by charge, can provide charge measurement which varies with the relationship to ground. Hence, the electric fields generated by charges on the wafer will vary from tool to tool and even vary at a particular tool if the location at which the robot is stopped cannot be accurately controlled. For that reason, this method promotes making relative rather than absolute measurements for comparing one static charge control solution in a particular tool with another solution in the same tool. Additionally, the process must stop to make the measurement, and that may increase process time or adversely affect the process as unacceptable consequences. Locating electrostatic sensors within process equipment is difficult due to space requirements and measurement interactions with moving equipment. Measurements with a fieldmeter are often inaccurate due to the precision required in the measurement distance, the variations in the boundary conditions (i.e. location of grounded surfaces) due to moving robotics, and only one side of the wafer can be measured at one time.
A second method of measuring the charge while the wafer is stored in the FOUP does not interfere with the process, but measurements may not be accurate because the charge may have time to dissipate. Also, sensors inside the FOUP can measure only a small area and may not accurately characterize the charge distribution on a 300 mm wafer.
In accordance with one embodiment of the present invention charge measurements are made as the wafer is moved into and out of the FOUP for greater measurement accuracy. This method provides a well-defined ground plane to fix the boundary conditions and allow the measurements to be compared from tool-to-tool as the FOUP containing the wafers being processed is moved from tool to tool. Wafer processing is not stopped and static charge may not have time to dissipate. Further, by digitizing and recording the output of the fieldmeter sensors, many readings of charge on the surface of the wafer can be recorded and mapped over much of the surface of the wafer as the wafer enters the FOUP.
To obtain the most precise measurements possible, fieldmeter sensors or other charge-measuring sensors are installed near the front door of the FOUP to measure electric field associated with charge on wafers in motion as they enter the FOUP. Sensors are positioned on opposite sides of the wafer to make simultaneous electric field measurements on both sides of the wafer. The entire wafer passes between the sensors. Fieldmeter sensors and modified fieldmeter circuit boards provide improved response time of the fieldmeters that are attached to metal disks positioned above and below a wafer to measure the electric fields on both sides of a wafer in motion into or out of the FOUP. The grounded conductive disks fix the geometry and thus establish an accurate relationship between measurement values and the charge on the wafers.
To avoid interfering with the tool process, the sensors are stationed on metal disks above and below the wafer as it passes in and out of the FOUP. The sensors scan the wafer along a linear segment or stripe across an off-center segment of the wafer as it moves into and out of the FOUP to measure charge at numerous points on the passing wafer. Precise measurements along the surface of the wafer may be determined knowing the distance between the sensor and the wafer (typically, about one inch), combined with the solid angle of sensor sensitivity and, hence, the measured area of the sensor zone as the wafer moves into and out of the FOUP. In one example, the lengthwise measurement angle of a sensor is 44xc2x0 @3 dB, and the cross-wise measurement angle of such sensor is 54xc2x0 @3 dB, and are thus capable of scanning about one-third of the area on each side of a wafer. The two sensors are located off center to avoid scanning the end effector of a robotic handler. Conventional charge sensors are modified to enhance speed of response to facilitate taking about 20 measurements along a diametric segment (or a chord segment displaced from the diameter) of a wafer being moved by a robot into or out of a FOUP.